Basically this means the path to the root bridge could change during the ISSU process while the state machine is being saved and copied to the other CPU. I have 2 5020 VPC'ed up to 2 Cat6K and see the same message. The workaround I use it to shut down the port channel on the non-root bridge C6K. After the ISSU is complete on both 5020, I re-enable the port-channel. Assume that each C6K is Etherchanneled to both 5020.

I do not know your topology but it looks like one of the ISSU prerequisite are not met to continue the upgrade procedure. I recommend issuing the command “show spanning-tree issu-impact” that will tell you if all ISSUE criteria’s are met.

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I do not know your topology but it looks like one of the ISSU prerequisite are not met to continue the upgrade procedure. I recommend issuing the command “show spanning-tree issu-impact” that will tell you if all ISSUE criteria’s are met.

Basically this means the path to the root bridge could change during the ISSU process while the state machine is being saved and copied to the other CPU. I have 2 5020 VPC'ed up to 2 Cat6K and see the same message. The workaround I use it to shut down the port channel on the non-root bridge C6K. After the ISSU is complete on both 5020, I re-enable the port-channel. Assume that each C6K is Etherchanneled to both 5020.

Single 10Gb link back to the 6509 was shutdown and this allowed the upgrade to complete. As the switch firmware upgrade reflected standard outage within the upgrade window, this was no impact to what was already within scope of the upgrade.

Thanks for your help. I did poke around with the STP "show spanning-tree issu-impact" and did not understand why it had an issue with the single 10Gb link. No other switch was tiered from the 6509 directly that also has a link into the 5010.

The only other link from the 5010 to the 6509 in any form was the managment port from the 5010. That is where I assume it was causing the issue.

The "show spanning-tree issu-impact" just sees that you don't have any downstream STP enabled ports. If the N5K is booted, that would cause problems as it will not be able to generate BPDU packets anymore during boot and the downstream switch would start forwarding on blocked ports, creating loops.

Basically, the N5K can only see root ports (upstream) and edge ports downstream (edge ports don't run STP, they have bpduguard enabled and don't generate BPDUs). That is why ISSU is possible: the switch just freezes its forwarding tables and the lack of BPDU generation is sure not to create troubles in the network. You cannot have any STP topology changes during booting.

My guess for your error message is that it seems that the N5K sees that your STP topology is not stable (it receives a BPDU with a TCN enabled). Booting or upgrading in a non-stable STP environment would also be dangerous, therefore your ISSU upgrade is cancelled, which seems logical.